2 5 8 



THE TANGANYIKA PROBLEM. 



long and simple. The radular dentition is characteristic, 

 and very strongly developed. A single row of teeth is 

 represented on page 257. It will be at once seen that the 

 characters of these teeth are highly suggestive of those of 

 a number of well-known forms. In gross detail the radular 

 dentition is very similar to that occurring in various forms 

 of Anchylotus figured by Troschel. The predominant 

 denticle on the admedian tooth is well developed in zonata, 

 as indeed it is in a very large number of dissimilar forms ; 



siip. int. g. 



P-g- 



Fig. 41. — Nervous system of Spekia zonata. a. from above ; b. from 

 the side ; b.g. buccal ganglion ; e.g. cerebral ganglion ; sup. int. g. 

 supra intestinal ganglion ; p.g. pedal ganglion. 



and an exactly analogous and widely prevalent feature is 

 presented in the difference of size and character between 

 the denticles on the heads of the outer and inner lateral 

 teeth. Perhaps, however, the most notable feature which 

 the radula presents is the peculiar structure of the median 

 tooth. The outer surface of this tooth is concave, like the 

 median tooth in Anchylotus, Tkiara, Melania brevis (Dorb.), 

 and Melanopsis. But it differs from all these forms in 

 having no predominant median denticle, there being instead 

 two lateral predominant denticles and a median concavity. 

 The only forms which appear to possess this peculiarity of 



